1. Field of the Invention
The present invention relates to an apparatus and a method for loading and storing lead frames such as are employed for the physical support and electrical connection of semiconductor devices. In particular, the present invention relates to a lead frame casing mechanism that aligns and immobilizes lead frames within the casing against a reference plane, for prevention of damage to the lead frames and ease of unloading by automatic mechanisms as employed in the art.
2. State of the Art
Integrated circuit (IC) semiconductor dies, also termed "chips," are enclosed in packages that provide protection from hostile environments and enable chip electrical interconnection to printed-circuit boards and other components. Lead frames are widely used in the packaging of semiconductor dies, for physical support of the die and to provide conductors for the die to the exterior of the package, which may be a transfer-molded plastic package, a ceramic package, a so-called "glob top" package, or other package structure known in the art.
Included within each lead frame of a conventional design are a plurality of leads and a central die attach pad area, typically termed a paddle, tab or island. The die attach pad area may be downset from the plane defined by the leads. With such a design, when a die is attached on the pad area, the top or active surface of the die is substantially co-planar with the leads of the lead frame so that bond wire lengths between the bond pads on the active surface and the inner lead ends are minimized. With other conventional lead frame designs, the entire frame is co-planar, and longer bond wires are employed to connect the bond pads on the active surface of the die with the inner ends of the leads. Another variation of lead frame design is the so-called leads-over-chip, or LOC, design, which eliminated the central die attach area and adhesively suspends the die by its active surface from a number of leads extending over the active surface.
In practice, a plurality of laterally adjacent lead frames are etched or stamped from a strip of metal, the frames remaining secured together through the packaging or encapsulation step of the fabrication process. This intermediate structure is commonly known as a "lead frame strip." The number of lead frames fabricated within each lead frame strip varies, depending on the size of the individual lead frames and the equipment and materials for processing them to attach the dies, wire bond and encapsulate the devices.
Lead frames are often fabricated by a vendor to the semiconductor device fabricator, rather than by the fabricator itself. The lead frames are stamped or etched, as noted above, and then shipped to the fabricator for die attachment, wire bonding, and encapsulation. At a minimum, the lead frame strips may be bundled, wrapped in thin paper or plastic wrap, and placed into a shipping tray. Layers of paper or plastic may be interposed between strips for additional protection. Conventional shipping trays consist of a flat bottom, two sets of opposing sides, and an open top. Within the tray, one or more movable partitions may be provided for accommodating the adjacent bundles of lead frame strips. Alternatively, stuffing materials may be included within the tray to accommodate the lead frame strips.
U.S. Pat. No. 4,798,305 issued Jan. 17, 1989 to Laverty discloses a variety of prior art shipping trays, including a tray design with movable partitions to accommodate different sizes and quantities of lead frames.
U.S. Pat. No. 5,125,509 issued Jun. 30, 1992 to Takei et al. discloses a rectangular lead frame container including a container body and a snap-on lid, the container body optionally including fixed or rigid projections or protrusions from the side walls to abut a stack of lead frames in the container.
A more complex shipping tray, also termed a "magazine", is described in U.S. Pat. No. 5,310,055 issued May 10, 1994 to Chu. The Chu patent provides a magazine for automatically collecting lead frame strips dispensed from an automated lead frame quality inspection machine. The magazine includes a cover that is placed over the magazine to provide a packaging tray for shipment of the collected lead frame strips. A plurality of holes are positioned within the base of the magazine. The magazine holes are positioned to lie directly underneath index holes within the lead frame strips when the lead frame strips are stacked inside the magazine. To prevent movement of the lead frame strips within the magazine during shipment, pins are inserted vertically into the index holes of the lead frame strip and through the magazine holes. Similarly-aligned holes to receive the pin tops are also included in a shield covering the top lead frame strip to protect the lead frame stack from oxidation and contamination.
The prior art shipping trays and containers of Laverty and Takei do not provide positive alignment of the lead frame strips against a reference plane, and their construction dictates that a snug fit or abutment of the lead frame strips against the partitions or projections may damage the strips if they are forced into the volume provided in the tray or container. If some clearance is allowed between the partitions or projections and the strips, movement is thereby permitted and damage may also ensure in that situation.
The prior art tray of Chu utilizing indexing pins to immobilize a lead frame strip stack has a significant disadvantage from a production standpoint, since any change in the positioning of the index holes within the lead frame strips requires a change in the production mold for the magazine and shield. Additionally, the pin alignment mechanism requires removal of the pins in order to release the lead frame strips from the tray for pick up by a strip unloading apparatus.
Therefore, it would be advantageous to develop a lead frame casing that aligns at least one side of a vertical stack of lead frame strips against a common reference plane and prevents any motion of the lead frame strips within the casing during shipment to avoid damage, but which still permits easy release and removal of the lead frame strips from the casing. It would be a further advantage to develop a lead frame casing design which is adaptable to permit immobilization of lead frame strips having assorted lengths and widths as well as index hole patterns without significant design changes, the use of different-sized partitions, or additional parts such as indexing pins.